Air springs are used for load leveling and ride height control on cargo vehicles. Typically, air springs are used in combination with shock absorbers and are installed between the axle and the chassis. When the vehicle is unloaded, the vehicle chassis is supported by its shock absorbers and mainsprings. When the vehicle is to be loaded, the air springs are inflated and the chassis is raised relative to the axle to a desired ride height. Thus, when loaded, the vehicle is supported by its mainsprings, its shock absorbers and the air springs which cooperate to dampen relative motion between the chassis and the axles.
The pressure in each air spring must be adjusted to maintain the chassis level at the desired ride height, for example, during loading and unloading. This may be accomplished manually and automatically with the aid of ride height sensors, control valves and a source of compressed air.
It is generally desirable to load a cargo vehicle to the maximum gross load limit as set by the Interstate Commerce Commission and state regulations for common carriers. Weighing stations are available which certify that the gross weight of the load does not exceed the limit prescribed by law. Additionally, each vehicle may have specific load limits for each individual axle; for example, the front axle typically has a lower load limit than the drive axle. Thus it is possible that the gross cargo load of a vehicle may not exceed the legal limit, but may impose an excessive load on the front axle because of uneven load distribution. Improper payload distribution causes overload on critical vehicle components such as strut, frame and tires. Unequal load distribution can cause differential loading across each axle, which may cause an unsafe side sway condition which affects trailer stability and handling as well as imposing excessive torsion loads on the frame and excessive compression loads on a particular tire, its mainspring and strut support.
Accordingly, it is generally desirable to balance the cargo load axle to axle from front to rear and to balance the load across each axle from tire to tire. In addition to achieving the maximum legal payload, care must be taken not to exceed the load limit for each axle. Generally, the operator desires to minimize the loading time while achieving the maximum allowable payload during initial loading. That is, the operator desires to quickly load up to the legal limit and maximize his profit by utilizing the full hauling capacity of the trailer. While doing this, the operator also desires to reduce the maintenance costs caused by overloading and increase profit potential by minimizing maintenance downtime.
Uniform load distribution is made difficult since the size and weight cargo units may vary substantially. This can cause localized overloading from front to rear of the trailer and localized overloading from tire to tire across each axle. Such uneven load distribution can impose excessive torsion loading forces on the frame which may cause structural failure and accelerated wear on tires, springs, struts and electric drive motors which are subjected to localized overloading.
Moreover, some loads, although initially balanced, may shift during transit, thereby causing an unbalanced load condition. Moreover, cargo loads may become seriously unbalanced as a result of scheduled off-loading of cargo at different destinations.